Diagnostic procedures using 129Xe spectroscopy characterstic chemical shift to detect pathology in vivo

ABSTRACT

An in vivo non-invasive method for detecting and/or diagnosing a pathological condition using hyperpolarized  129 Xe spectroscopy is disclosed. Generally stated, the method includes determining the magnitude of spectral peaks which represent particular chemical shifts and comparing the observed magnitudes to those of healthy individuals. Preferably, the method includes subtracting substantial backgrounds and accounting for secondary conditions such as the polarization of hyperpolarized gas administered. Additionally, a quantitative analysis of hyperpolarized  129 Xe spectra advantageously allows, a physician to establish the extent of disease progression. Advantageously, this method can be used regardless of the method of hyperpolarized  129 Xe administration.

RELATED APPLICATIONS

This application claims the benefit of priority of U.S. ProvisionalPatent Application Ser. No. 60/217,971, filed 13 Jul. 2000, the contentsof which are hereby incorporated by reference as if recited in fullherein.

This application is a continuation of U.S. application Ser. No.09/904,343 filed Jul. 12, 2001, which claims priority to U.S.Provisional application No. 60/217,971 filed Jul. 13, 2000, the entiredisclosure which is hereby incorporated by reference herein as if havingbeen fully disclosed herein.

FIELD OF THE INVENTION

The present invention relates to magnetic resonance spectroscopy methodsutilizing chemical shifts of hyperpolarized ¹²⁹Xe.

BACKGROUND OF THE INVENTION

MRI using hyperpolarized noble gases has been demonstrated as a viableimaging modality. See e.g., U.S. Pat. No. 5,545,396 to Albert et al. Thecontents of this patent are hereby incorporated by reference as ifrecited in full herein. Albert et al. proposed several techniques ofintroducing the hyperpolarized gas (either alone or in combination withanother substance) to a subject, such as via direct injection,intravenous injection, and inhalation. See also “Biological magneticresonance imaging using laser-polarized ¹²⁹ Xe,” Nature, pp. 199-201(Jul. 21, 1994). Other researchers have since obtained relativelyhigh-quality images of the lung using pulmonary ventilation of the lungwith both hyperpolarized ³He and ¹²⁹Xe. See J. R. MacFall et al., “Humanlung air spaces. Potentialfor MR imaging with hyperpolarized He-3,”Radiology 200, 553-558 (1996); and Mugler et al., “MR Imaging andspectroscopy using hyperpolarized ¹²⁹ Xe gas: Preliminary human results”Mag Res Med 37, 809-815 (1997). See also E. E. de Lange et al, “LungAirspaces. MR Imaging evaluation with hyperpolarized Helium-3 gas,”Radiology 210, 851-857 (1999); L. F. Donnelly et al., “Cysticfibrosis:combined hyperpolarized ³ He-enhanced and conventional proton MR imagingin the lung—preliminary observations,” Radiology 212, 885-889 (1999);and H. P. McAdams et al., “Hyperpolarized ³ He-enhanced MR imaging oflung transplant recipients: Preliminary results,” AJR 173, 955-959(1999).

These researchers and others have investigated vascular and tissueimaging using inhaled or injected hyperpolarized gases to observe anddetect abnormalities in body cavities. ¹²⁹Xe may additionally be used todetect abnormalities within tissues because of its high solubility(relative to He) and lipophilic nature. Despite these advantages,hyperpolarized ¹²⁹Xe cannot readily or typically achieve the signalstrength readily attainable with hyperpolarized ³He. Hyperpolarized¹²⁹Xe has an inherently shorter lifespan even under the best ofconditions due to depolarizing interactions between ¹²⁹Xe nuclei. Whenhyperpolarized ¹²⁹Xe additionally interacts with body tissues, itslifetime is reduced further as will be discussed hereinbelow.

¹²⁹Xe can be administered to a patient by several means, such as byinhalation and injection. During inhalation delivery, a quantity ofhyperpolarized ¹²⁹Xe is inhaled by a subject (a subject breathes in the¹²⁹Xe gas) and the subject then holds his or her breath for a shortperiod of time, i.e. a “breath-hold” delivery. This inhaled ¹²⁹Xe gasvolume then exits the lung space and is generally taken up by thepulmonary vessels and associated blood or pulmonary vasculature at arate of approximately 0.3% per second. For example, for an inhaledquantity of about 1 liter of hyperpolarized ¹²⁹Xe, an estimated uptakeinto the body is about 3 cubic centimeters per second or a totalquantity of about 40 cubic centimeters of ¹²⁹Xe over about a 15 secondbreath-hold period. Accordingly, it has been noted that such uptake canbe used to generate images of pulmonary vasculature or even organsystems more distant from the lungs. See co-pending and co-assigned U.S.patent application Ser. No. 09/271,476 to Driehuys et al., entitled“Methods for Imaging Pulmonary and Cardiac Vasculature and EvaluatingBlood Flow Using Dissolved Polarized ¹²⁹Xe,” the contents of which arehereby incorporated by reference as if recited in full herein.

Many researchers are also interested in the possibility of using inhaled¹²⁹Xe for imaging white matter perfusion in the brain, renal perfusion,and the like. While inhaled delivery ¹²⁹Xe methods are suitable, andindeed, preferable, for many MR applications for several reasons such asthe relatively non-invasive characteristics attendant with such adelivery to a human subject, inhalation or ventilation-based deliveriesmay not be the most efficient method to deliver a sufficiently largedose to more distant (away from the pulmonary vasculature) target areasof interest. In addition, due to the dilution of the inhaled ¹²⁹Xe alongthe perfusion delivery path, relatively large quantities of thehyperpolarized ¹²⁹Xe are typically inhaled in order to deliver a smallfraction of the gas to the more distal target sites or organ systems.For example, the brain typically receives only about 13% of the totalblood flow in the human body. Thus, the estimated 40 cc's ofhyperpolarized ¹²⁹Xe taken up into the pulmonary vessels from the1-liter inhalation dose may be reduced to only about 5 cc's by the timeit reaches the brain.

Further, the hyperpolarized state of the gas is sensitive and can decayrelatively quickly due to a number of relaxation mechanisms. Indeed, therelaxation time (generally represented by a decay constant “To”) of the¹²⁹Xe in the blood, absent other external depolarizing factors, isestimated at T₁=4.0 seconds for venous blood and T₁=6.4 seconds forarterial blood at a magnetic field strength of about 1.5 Tesla. SeeWolber et al., Proc Natl Acad Sci USA 96:3664-3669 (1999). The moreoxygenated arterial blood provides increased polarization life over therelatively de-oxygenated venous blood. Therefore, for about a 5-secondtransit time, the time estimate for the hyperpolarized ¹²⁹Xe to travelto the brain from the pulmonary vessels, the ¹²⁹Xe polarization isreduced to about 37% of its original value. In addition, the relaxationtime of the polarized ¹²⁹Xe in the lung itself is typically about 20-25seconds due to the presence of paramagnetic oxygen. Accordingly, ¹²⁹Xetaken up by the blood in the latter portion of the breath-hold cycle candecay to about 50% of the starting polarization (the polarization levelof the gas at the initial portion of the breath-hold cycle). Thus,generally stated, the average polarization of the ¹²⁹Xe entering thepulmonary blood can be estimated to be about 75% of the starting inhaledpolarization value. Taking these scaling effects into account, thedelivery to the brain of the inhaled ¹²⁹Xe can be estimated as about 1.4cc's of the inhaled one liter dose of ¹²⁹Xe polarized to the samepolarization level as the inhaled gas (0.75×0.37×5 cc's). This dilutionreduces signal delivery efficiency; i.e. for remote target areas (suchas the brain), the quantity of delivered ¹²⁹Xe signal is typicallyseverely reduced to only about 0.14% of that of the inhaled ¹²⁹Xe. SinceMR imaging requires high signal strength to achieve a clinically usefulspatial resolution in the resulting image, inhalation delivery may notproduce clinically desirable images of distal or remote target organs orregions. However, much smaller quantities, for example on the order ofapproximately 0.01 cc's of ¹²⁹Xe, polarized to about 10%, are sufficientto provide signal information for MR spectroscopy.

An alternative method for delivering hyperpolarized ¹²⁹Xe is injection.¹²⁹Xe injection can be accomplished by suspending the hyperpolarized gasin a carrier or by direct gaseous injection. See international patentapplication PCT/US97/05166 to Pines et al, the contents of which arehereby incorporated by reference as if recited in full herein. In thisapplication, Pines et al describes suitable injectable solutions inwhich to suspend hyperpolarized gases for in vivo use to effectivelytarget regions or areas of the body. See also co-pending U.S. patentapplication Ser. No. 09/804,369 to Driehuys et al., entitled “DiagnosticProcedure Using Direct Injection of Gaseous Hyperpolarized ¹²⁹Xe andAssociated Systems and Products,” the contents of which are herebyincorporated by reference as if recited in full herein. Generallystated, this patent application describes methods and an associatedapparatus for injecting hyperpolarized ¹²⁹Xe directly into thevasculature. The gas is preferably delivered such that the gassubstantially dissolves into the vasculature proximate to the injectionsite or alternatively resides in the bloodstream for a period of time.As also discussed therein, surfactants may preferably additionally beadded to facilitate the dissipation of injected bubbles.

Spectroscopy using hyperpolarized ¹²⁹Xe is advantageous because of thedocumented sensitivity of ¹²⁹Xe to its environment and the comparativelylow levels of hyperpolarized ¹²⁹Xe signal attainable (due to bothenvironmental factors and the inherent properties of ¹²⁹Xe compared tohyperpolarized ³He). By nature, spectroscopy requires a much smallersignal density because high spatial resolution is not required.Nonetheless, important information can be garnered from hyperpolarized¹²⁹Xe spectroscopy. Many researchers have investigated characteristicchemical shifts observed when hyperpolarized ¹²⁹Xe comes into contactwith different tissues, as seen in Table 1. As shown, large frequencyshifts (on the order of 200 parts per million or “ppm”) from free gasphase (referenced at 0 ppm) have been observed. This frequency shift isfar greater than that observed with proton spectroscopy (generallystated, at most about 5 ppm). Therefore, spectroscopy is a modalitywhich may be particularly suited to capitalize upon the behavior ofhyperpolarized ¹²⁹Xe.

TABLE 1 Characteristic shifts from free gaseous hyperpolarized ¹²⁹Xe(referenced at 0 ppm) of hyperpolarized ¹²⁹Xe when exposed to differenttissues. Tissue ppm Reference Water 191.2 Wilson 99 Epicardial fat 192Swanson 99 Brain, lipid rich 194 Albert 99 Brain tissue 194.5 Swanson 97Plasma 195.6 Wilson 99 Brain 198.0 Wilson 99 Lung parenchyma 198.6Wilson 99 Brain tissue 199 Swanson 99 Kidney 199.8 Wilson 99 Brain -lipid poor 201 Albert 99 Liver 201.8 Wilson 99 T. Californica membrane209 Miller 81 RBC (oxygenated) 213.0 Wilson 99 RBC (de-oxygenated) 216.0Albert 99

All of the studies tabulated above involve healthy tissues. However,because ¹²⁹Xe is so sensitive to its environment, characteristics ofdiseased states can also be sensed with ¹²⁹Xe spectroscopy. For example,Wolber et al., in “In vivo hyperpolarized ¹²⁹Xe spectroscopy in tumors,”Proc Int'l Mag Reson Med 8, 1440 (2000), suspended hyperpolarized ¹²⁹Xein perfluorooctyl bromide (PFOB) or saline and injected it intosubcutaneous tumors grown in rats. Because Wolber et al. suspended ¹²⁹Xein a carrier fluid, the resultant signal spectrum was likely tainted orinfluenced by the carrier fluid. For example, the signal from the ¹²⁹Xein the saline may have substantially obscured the peak of interest (i.e.the peak reflecting the ¹²⁹Xe in the tumor tissue).

However, these experiments provided very little in the way ofquantifiable information. Diseases of interest often cannot be diagnosedmerely by the appearance of peaks denoting characteristic chemicalshifts, since healthy tissues may also exhibit the same characteristics(e.g., some lipid is expected, but an excess or reduced amount of lipidmay be problematic). In view of the foregoing, there remains a need forimproved methods to determine the presence of certain diseases and/orpathological conditions as well as the extent or progression of thedisease or condition and/or other quantitative information.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to detect anddiagnose pathological conditions in vivo utilizing characteristicchemical shifts of hyperpolarized ¹²⁹Xe.

It is an additional object of the present invention to provide a methodfor quantifying the extent of a pathological condition.

It is a further object of the present invention to provide a method forquantification of pathological states in vivo, in a manner which candecrease the impact of certain potentially data corrupting parameterssuch as variations in polarization and the volume of hyperpolarized gasadministered to a subject.

It is another object of the present invention to filter or suppressundesirable background information from the spectroscopic signalassociated with the hyperpolarized gas, thereby providing signalcharacteristics associated with a physiological or pathologicalphenomenon of interest in vivo.

One aspect of the present invention is directed toward a method fordetecting pathology using hyperpolarized ¹²⁹Xe spectroscopy. This methodinvolves administering a bolus of hyperpolarized ¹²⁹Xe to a patient andtransmitting an RF pulse to a region of interest. An NMR RF excitationcoil positioned proximate the region of interest can be used to transmitand receive the signal(s). In addition, localizing gradients can beapplied as needed in the presence of the RF pulse as is well known tothose of skill in the art. The response of the hyperpolarized gas to theRF pulse is received such that spectral peaks of interest can beidentified and analyzed. The spectral peak may be further evaluated orquantified, and/or normalized. A pathological condition can then bedetected on the basis of comparing the spectral peaks with a standardspectrum.

In certain embodiments, the polarization of the ¹²⁹Xe prior toadministration as well as the volume of gas administered can beaccounted for. Alternatively, the spectral peak of interest can benormalized by another selected spectral peak, such as the dissolvedphase-plasma peak or dissolved phase RBC (red blood cell) peak.

In certain embodiments, where the pathological condition is adegenerative disease, the stage or progression, remission, or remedialstate of the disease can be determined by taking a signal of a dose ofhyperpolarized gas administered to a subject in vivo (a) at a first timeand (b) at a second subsequent time (such as at selected intervalsand/or before and after treatments). As such, the methods of the presentinvention can be used to monitor the progression of a disease and theefficacy of a treatment regimen.

In embodiments of the present invention, ¹²⁹Xe signal data associatedwith the interaction of ¹²⁹Xe and non-targeted tissues can be filteredout by employing selected RF pulse sequences. For example, NMR signalscan be filtered on the basis of one or more of T₁, T₂, T₁ρ, T₂*,diffusion coefficient, and velocity (of the blood).

Another aspect of the present invention is directed toward a method ofdetecting atherosclerosis in the coronary arteries. This method involvesadministering a bolus of hyperpolarized ¹²⁹Xe gas to a patient,delivering at least a portion of the administered hyperpolarized ¹²⁹Xegas to a region of interest, applying at least one resonant RF pulsesequence to the region of interest, acquiring and analyzing at least oneNMR response signal (associated with the ¹²⁹Xe), and determining thepresence of atherosclerotic plaques on the basis of the analyzedresponse signal.

In certain embodiments, the method can also include taking a backgroundspectrum of the heart (a spectrum of the polarized ¹²⁹Xe in the blood ofthe chambers/vessels of the heart), which can be subtracted from theacquired signal spectra to accentuate or amplify the signal of thehyperpolarized ¹²⁹Xe in the tissue (vessel wall, plaque, or otherbiosubstance or analyte of interest). The signal acquisitions cancarried out responsive to a cardiac event and the flip angles of theexcitation pulse(s) or pulse sequence(s) can be chosen such that they donot destroy all the polarization of the gas within the heart itself.Alternatively, signals of the carotid arteries may be used as indicatorsof the health of the coronary arteries, which can allows larger flipangles and therefore a better SNR (signal-to-noise ratio) over otherregions.

Because of the comparatively low signal of ¹²⁹Xe (compared to ³He) andits solubility in lipids, ¹²⁹Xe spectroscopy may be particularlysuitable for obtaining in vivo pathologic information on certaininternal locations over ¹²⁹Xe imaging. For example, because ¹²⁹Xe isfurther sensitive to its environment, spectroscopy using ¹²⁹Xe may beused as a sensitive probe for diseased states, such as evaluating invivo an increased lipid content characteristic of arteriosclerosis oraltered cells such as those characteristic of tumors, plaques or otherabnormalities.

Embodiments of the present invention are therefore directed toward amethod of non-invasively or minimally invasively probing tissues in vivoto detect pathological or abnormal conditions. The hypersensitivity of¹²⁹Xe to its environment, when used according to the methods of thepresent invention, can advantageously allow a physician or a programmeans to quantitatively and/or qualitatively assess the presence and/orextent of a diseased condition. The hyperpolarized gas can beadministered via any desired method including, for example, inhalationand injection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the human circulatory systemillustrating the venous and arterial portions thereof. The deoxygenatedblood is represented by the lighter/white regions and the oxygenatedblood is represented by the darkened regions.

FIG. 2 is a screen printout of a set of brain spectra of a healthy humantaken a few seconds after inhalation of 500 cc's of ¹²⁹Xe polarized toabout 2%.

FIG. 3 is a flow chart depicting the chain of events followed todiagnose pathological conditions according to the present invention.

FIG. 4 is graph of left ventricular pressure correlated in time withventricular volume and an electrocardiogram for a complete cardiaccycle.

FIG. 5 is a flow chart depicting a procedure for detecting ¹²⁹Xe spectrafrom the coronary arteries according to the present invention.

FIG. 6 is a perspective view of an apparatus for determining the extentof polarization for a sample of hyperpolarized gas.

FIG. 7 is a flow chart demonstrating one method of normalizing spectraaccording to the present invention.

FIG. 8 is a flow chart depicting an alternative method for normalizingspectra according to the present invention.

FIG. 9A is a simulated spectrum denoting spectral peaks due to blood,plasma, fatty plaques, and free ¹²⁹Xe gas.

FIG. 9B is a simulated spectrum under the same conditions as FIG. 9A,but acquired with a T₂-weighted pulse sequence according to the presentinvention.

FIG. 10A is a simulated spectrum of the coronary arteries of a personwho has significant fatty plaques indicative of atherosclerosis.

FIG. 10B is a simulated spectrum of an individual who does not havesignificant detectable coronary artery plaques. This spectrum istherefore representative of a “reference” or “standard” spectrum.

FIG. 11A is a simulated background spectrum of an individual where theheart chambers contain substantial amounts of hyperpolarized ¹²⁹Xegas/blood mixture but the coronary arteries are substantially free ofhyperpolarized ¹²⁹Xe.

FIG. 11B is a simulated spectrum of an individual's heart afterhyperpolarized ¹²⁹Xe is in both the heart chambers and the coronaryarteries.

FIG. 11C is a simulated corrected spectrum of the spectrum shown in FIG.11B minus the background spectrum of FIG. 11A.

FIGS. 12A and 12B are graphs of a spectrum of an NMR signal of an invitro sample of an aortic wall of a 55 year-old deceased human maleexposed to thermally polarized ¹²⁹Xe at 25 bars of pressure. FIG. 12Aillustrates the signal obtained for healthy tissue and FIG. 12Billustrates the signal obtained for plaque.

FIGS. 13A and 13B are graphs of a spectrum of an NMR signal of an invitro sample of an aortic wall of a 70 year-old deceased human femaleexposed to thermally polarized ¹²⁹Xe at 25 bars of pressure. FIG. 13Aillustrates the signal obtained for healthy tissue and FIG. 13Billustrates the signal obtained for plaque.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout. In the figures, certainlayers, regions, features or components may be exaggerated or enlargedfor clarity.

As known to those of skill in the art, polarized gases are collected,frozen, thawed, and used in MRI applications. For ease of description,the term “frozen polarized gas” means that the polarized gas has beenfrozen into a solid state. The term “liquid polarized gas” means thatthe polarized gas has been or is being liquefied into a liquid state.The term “gaseous hyperpolarized ¹²⁹Xe” indicates the gaseous phase ofthe “hyperpolarized ¹²⁹Xe gas.” Thus, although each ¹²⁹Xe term includesthe word “gas,” this word is used to name and descriptively trackhyperpolarized noble gas produced via a hyperpolarizer to obtain apolarized “gas” product. Thus, as used herein, the term “gas” has beenused in certain places to descriptively indicate a hyperpolarized noblegas product and may be used with modifiers such as solid, frozen, andliquid to describe the state or phase of that product at a particularpoint in time (such as at administration or during accumulation). U.S.Pat. No. 5,809,801 to Cates et al. describes a cryogenic accumulator forspin-polarized ¹²⁹Xe. U.S. Pat. No. 6,079,213 to Driehuys et al.,entitled “Methods of Collecting, Thawing, and Extending the Useful Lifeof Polarized Gases and Associated Accumulators and Heating Jackets”,describes an improved accumulator and collection and thaw methods. Thedisclosures of these documents are hereby incorporated by reference asif recited in full herein.

As used herein, the terms “hyperpolarize,” “polarize,” and the like meanto artificially enhance the polarization of certain noble gas nucleiover the natural or equilibrium levels. Such an increase is desirablebecause it allows stronger imaging signals corresponding to better MRIand spectroscopy of the substance and a targeted area of the body. As isknown by those of skill in the art, hyperpolarization can be induced byspin-exchange with an optically pumped alkali-metal vapor oralternatively ³He can be polarized by metastability exchange. See Albertet al., U.S. Pat. No. 5,545,396. Other methods may also be used, such asdynamic nuclear polarization (“DNP”) and “brute force” methods whichpropose to cool the ³He or ¹²⁹Xe to very low temperatures and thenexpose them to very high magnetic fields to enhance the thermalequilibrium polarization.

As discussed hereinabove, hyperpolarized ¹²⁹Xe can be administered to apatient by inhalation or injection. If the administration modality isinjection, ¹²⁹Xe can be suspended in a carrier fluid or injecteddirectly such as in gaseous form. However, regardless of what tissue isof interest, if the ¹²⁹Xe is suspended in a carrier fluid, it is likelythat the carrier fluid itself distorts the results of the spectra and/orsubstantially obscures a spectral peak of interest. The carrier fluidmay also react with the target tissue (region of interest) and/orpotentially produce compounds with molecules in or around the tissue ofinterest, which may thereby cause the chemical shift of hyperpolarized¹²⁹Xe to differ from that which would be observed with merely the tissueof interest and hyperpolarized ¹²⁹Xe. Therefore, direct injection ofgaseous ¹²⁹Xe or administration via inhalation may be particularlysuitable for certain embodiments or applications.

The present invention recognizes that hyperpolarized ¹²⁹Xe is asensitive probe for its environment. Specifically, spectroscopyutilizing hyperpolarized ¹²⁹Xe is capable of detecting pathologicalconditions because of the frequency shift inherent in the response of¹²⁹Xe to its environment. The frequency shift observed with a tissue ischaracteristic of the tissue type and not significantly differentbetween individuals. However, some variations between people based onrace, gender and/or age are expected. These differences are due to thefact that some tissues typically vary in composition (e.g., bones becomeless dense with age). Typically a range of values characteristic ofhealthy tissues is expected. This range may be determined by largeepidemiological studies. Alternatively, a “healthy” (substantiallynon-diseased or diseased to a lesser extent) standard may be acquiredfrom an individual at an early stage (or at a different position orlocation in the body away from the diseased or abnormal target region).For the former, subsequent values acquired later in life or aftercertain treatments can be compared to this earlier standard. The term“pathological condition” refers to a biophysical structure orbiochemical state or condition of a cell or cells, tissue, organ, orsubstance in the body. As used herein, the term includes healthypathological conditions (i.e., the absence of overt pathology) as wellas pathological conditions associated with diseases of the body whichproduce changes in structure or function and/or abnormal or progressivedisorders.

Since different cell and tissue conditions can cause characteristicshifts, an exposure of hyperpolarized ¹²⁹Xe to the human body mayproduce a characteristic pattern that demonstrates a superposition ofthe ¹²⁹Xe frequency shifts apparent with each of the cell and tissueconstituents. In healthy tissues, the location of spectral peaks (i.e.chemical shifts) and/or their sizes (as measured by spectral peakmagnitudes and/or areas) may be different than when the cells comprisingthe tissues are altered by a pathological condition. For example, thefatty plaques indicative of late stage atherosclerosis or fatty unstablelesions of plaque may produce a respective “signature.” As used herein,the term “signature” refers to a spectroscopic signal forming acharacteristic pattern of a spectral peak or peaks at designatedchemical shifts which may differ in size, shape, and/or location fromthat of a healthy or non-diseased state. With proper quantification ofthe signature, it may even be possible to distinguish between the earlystages of a condition from later more developed pathology, even if thepathological tissue profile remains essentially the same throughout thedisease progression but changes in cell proliferation or number, e.g.,tumor size. Diagnostic in vivo tools such as these may enable earlydetection in disease progression, which may be successfully used in massscreening procedures, preferably even before potentially debilitatingsymptoms occur, and therefore allow for earlier intervention andconsequentially potentially more effective treatment or even treatmentwhich may prevent or delay the onset of symptoms and/or the diseaseitself.

Thus, by utilizing in vivo obtained signals of ¹²⁹Xe spectrum“signatures,” diseased states may be effectively identified, detectedand/or diagnosed. Therefore, hyperpolarized ¹²⁹Xe can be used in vivo asa non-invasive (or minimally invasive, if administered via injection)probe for pathological conditions. FIG. 1 is a schematic representationof a circulatory system. After hyperpolarized ¹²⁹Xe is administered,e.g., via inhalation or injection as described hereinabove, thehyperpolarized ¹²⁹Xe will travel throughout the body via the circulatorysystem until it is absorbed by a tissue or becomes completelydepolarized.

Therefore, for example, when hyperpolarized ¹²⁹Xe is delivered viainhalation, the ¹²⁹Xe is absorbed by the blood in the alveolarcapillaries. The Xe-blood mixture then is pumped via the pulmonary veininto the left atrium of the heart. The mixture then flows into the leftventricle and to the periphery and on into the region of interest(unless the region of interest is located within the pulmonary region)where MR spectroscopy can be accomplished. Obviously, if the pulmonaryregion is of interest, spectroscopic data can be acquired before thegas/blood mixture migrates to the heart.

This method may be used to detect pathological conditions such as, butnot limited to, amyloid plaques associated with Alzheimer's disease,demyelination associated with multiple sclerosis, tumors, thrombi, andatherosclerotic plaques. In fact, any disease which has a characteristichyperpolarized ¹²⁹Xe signature due to a change in the biophysical orbiochemical state of a cell or cells which comprise the region ofinterest may be advantageously detected with this method.

One potentially important organ of interest for this type of analysis isthe brain. One benefit of brain spectra is that the resulting spectrawill not be substantially altered by factors such as blood flow, sincebrain perfusion is relatively constant despite environmental variablessuch as hormonal activity, sympathetic nerve activity and arterial bloodpressure. FIG. 2 shows a set of brain spectra taken a few seconds afterinhalation of 500 cc's of 2% polarized ¹²⁹Xe. This healthy braindemonstrates a clear definable pattern of four distinct peaksrepresenting the interaction of ¹²⁹Xe with the differenttissues/substances it is in contact with, such as blood cells, plasma,white matter, and grey matter.

As stated hereinabove, the observed spectra represent the environment inthe brain and its interaction with ¹²⁹Xe. As Table 1 shows, specificpeaks can be assigned to specific environmental conditions, i.e., thepresence of a particular cell and/or tissue type. Advantageously,therefore, noting the presence of a spectral peak not apparent inhealthy tissues can be useful to detect certain pathological conditionscharacterized by unique cells and/or non-cellular substances such astumors and plaques, where a characteristic change in tissue compositiondefines the pathological state.

In many cases, it would be of additional interest to quantify theprogression of the diseased state, thereby giving an indication of theextent of damage to the tissue, for example, to differentiate apea-sized tumor from a fist-sized tumor. However, some pathologicalconditions are identified as diseased predominantly by the excess orreduction of a signal associated with a healthy condition, whichindicates that the observed chemical shifts may be similar in locationin both “healthy” (substantially non-diseased) tissues compared totissues with diseased conditions, but differ in peak magnitude, lineshape (the line shape may be wider, lower, or present a differingspectral profile about the peak or peaks of interest), or area under theline associated with the peak. Therefore, in certain embodiments,differences in external factors that are not of interest, such as theinitial polarization of the ¹²⁹Xe, are taken into account to makeaccurate quantification possible. Because spectroscopy is quantitativein that the spectra have associated sizes (displayed by the profile ofthe line such as the height or magnitude of the peak and/or area underthe spectral peaks), the ratio of the magnitudes of (or alternatively,the ratio of the areas within) one or more spectral peaks may becalculated and compared to evaluate or indicate a pathologicalcondition. However, the parameter used to quantify events of interestmust be chosen carefully. As known to those of skill in the art, thearea contained within a spectral peak representing a chemical shift ofinterest may be a preferred parameter with which to quantify thechemical shift. This is because under some conditions (depending onexchange parameters), spectral peaks tend to be broad and short whereasunder other conditions the spectral peaks are narrow and tall.Therefore, the magnitude of a spectral peak may not represent acondition accurately, whereas the area may be a more reliable parameter.

Regardless of what parameter is used to quantify spectral peaks,normalizing the spectral data may advantageously allow improvedquantification of the condition of the patient. The term “normalizing”means to adjust the signal data of the spectral peak or peaks ofinterest to account for selected signal variables. This adjustment mayinclude using the mathematic ratio of the values of certain peaksassociated with selected known biomatter (RBC, plasma, etc) within theresponse spectrum to quantify the hyperpolarized gas signal in theregion of interest. The adjustment may include using the polarizationlevel (and/or quantity) of the administered gas to that used to obtainthe reference spectrum to quantify the magnitude of the signal. As such,the normalization can use relative data and/or absolute data. Forexample, the ratio of the spectra for the blood to spectra of the braintissue (the ratio of the magnitude or area of selected spectral peaks)can be calculated. Of course, other known chemical shift peak locationscan also be used to normalize the value of the spectra peak of interest.The absolute data can include data associated with the polarizationlevel of the gas as it is delivered to the patient and/or the amount ofgas administered thereto (to account for signal strength).

For ¹²⁹Xe NMR spectroscopy, spectral peaks at 213.0 ppm and 216.0 ppm(which usually cannot be easily resolved) represent oxygenated andde-oxygenated blood, respectively, according to Albert and Wilson (seeTable 1). Additionally, brain tissue is typically measured around 197ppm. During analysis, if the ratio of the signal for blood to braintissue departs substantially from that which is considered “healthy” or“normal”, for example, the spectra can indicate that a problem existswith perfusion (such as restricted blood vessels due to chronichypertension). Alternatively, higher ratios of lipids in the cardiac orcerebral region (compared to dissolved phase ¹²⁹Xe in the blood, forexample) may indicate atherogenesis.

In practice, a diagnostic or screening procedure utilizing the instantinvention can occur as shown in FIG. 3. A bolus of hyperpolarized ¹²⁹Xeis administered to a patient in gas or liquid (or suspended in a liquid)form (Block 300) by injection or inhalation. The ¹²⁹Xe is delivered to atarget area (region of interest) via the circulatory system (Block 310).A region-specific NMR coil positioned over the region of interesttransmits an RF pulse sequence (Block 320) and receives a FID (Block330). As noted above, localizing gradients can also be applied about theregion of interest so as to localize the resonance region. For example,localizing gradients can be applied so that a single one of the coronaryarteries is excited (either the left or right). In any event, theFourier Transform of the acquired data is then calculated (Block 340).

The transformed signal data can be further processed (Block 350) whichmay include, but is not limited to, one or more of subtractingbackground noise, filtering undesirable signal data (such as thoseportions of the signal or spectra attributed to carrier liquids ordeposits in non-target tissue or blood and the like), determining thefrequency shift and size of the shift for any number of peaks withinpre-determined ranges in the spectrum, and normalizing the data such asfinding the ratios between magnitudes and/or areas of different spectralpeaks within the response spectrum or accounting for polarization leveland amount of polarized gas delivered to the subject. The processed datacan be visually displayed (Block 360). In any event, a clinician orphysician, computer, or program code can compare the signal data fromthe patient to that of a reference standard (Block 370) to diagnoseand/or detect a pathological condition or absence thereof. Thecomparison can analyze whether there are additional peaks or missingpeaks from a norm or whether there are dissimilar sized peaks andestablish the correlation to what pathological condition may beindicated. The reference standard can be spectra generated fromcorresponding hyperpolarized gas NMR spectroscopic response signals ofone or more “healthy” subjects or based on a historical (signal obtainedin the past) response signal(s) of the patient undergoing analysis.

Typical spectra standards for target tissues, regions, or pathologicconditions may vary depending on certain parameters in populationsegments such as age, gender, and/or race. For example, the compositionof some tissues differs with age and gender. Most likely, astatistically valid range of “healthy” or undiseased characteristicsestablished by an epidemiological study can be determined and used as areference standard, and deviants outside this range or on the outeredges of this range can be monitored closely for and/or notified of anobserved or potential pathological condition. Alternatively, asmentioned hereinabove, an early set of spectra can be acquired for anindividual and spectra acquired later in life (or after and treatments)can be compared to the earlier values to provide an internalindividual-specific standard. Advantageously, an individual-specificstandard additionally enables monitoring disease progression andtreatment efficacy.

Atherosclerosis in coronary arteries is one condition for which in vivo¹²⁹Xe spectroscopy has a great potential to make a significant impact inearly detection of pathology. Atherosclerosis is a multi-stageprogressive disease, in which only the late stages are characterized bynoticeable circulatory compromise, calcified lesions, and thrombosis.Often, if the disease is only detected at these later stages, thecondition's progression cannot be reversed or even retarded, which mayresult in a significant reduction in the patient's quality of life.Early stages of atherosclerosis are characterized by fatty streaks whichcomprise lipid-filled foam cells in the intima and very littleextracellular lipid. Generally the foam cells are of macrophagederivation, although some are from smooth muscle cells. As the diseaseprogresses, lipid, collagen, and proteoglycans accumulate in theextracellular matrix and fibrous plaques form. Macrophages accumulate inthe arterial wall and are captured there, oxidizing and accumulatingoxidized low density lipoprotein (LDL). Later stages of the diseaseinvolve smooth muscle cell and collagen accumulation of LDL as well, andfully developed lesions eventually develop and protrude into the lumen,thereby reducing blood flow to downstream tissues. Blood flow can becompletely blocked, causing platelets to adhere and risking embolism andthrombosis. Complications from atherosclerosis are believed to be theleading cause of death in the United States and Europe. Early detectionof this disease, long before fully developed lesions form, would beextremely beneficial for improved treatment regimens.

One critical location where atherosclerosis is common is in the coronaryarteries. One obvious difficulty in trying to investigate this area withhyperpolarized ¹²⁹Xe is that the ¹²⁹Xe in the blood (both associatedwith blood cells and in dissolved phase in the plasma) within the heartchambers creates a much stronger signal than the ¹²⁹Xe of interestwithin the much smaller coronary vessels.

One technique for addressing this difficulty includes taking abackground spectrum which represents a heart “full” of polarized ¹²⁹Xeand blood (yet none in the coronary arteries) and subtracting thisbackground from all subsequent spectra. As shown in FIG. 4, one way toachieve this subtraction technique is through cardiac gating. Since verylittle blood goes directly from the heart chambers into the myocardium,the majority of the blood nourishing the myocardium exits the leftventricle through the aorta and into the coronary arteries. Therefore,it is only after left ventricular contraction (i.e., systole) that the¹²⁹Xe enters the coronary arteries and myocardium. Thus, in a preferredembodiment, the background spectrum is acquired before ventricularsystole, so that the background scan can be used to represent a “full”heart with polarized ¹²⁹Xe and blood to enhance the signal in thecoronary vessels. For further discussion of exemplary background andcardiac gating methods, see co-pending U.S. application Ser. No.09/271,476, the contents of which are hereby incorporated by referenceas if recited in full herein.

As shown in FIG. 4, the ventricle is full and fairly constant in volumeduring atrial systole and isovolumetric contraction (see the middlegraph). Therefore, the background scan can be acquired anytime duringthis interval.

Advantageously, subsequent spectra can be obtained at substantially thesame point in the cardiac cycle to reduce potential effects due todifferent blood volumes in the heart between background and subsequentscans. Therefore, subsequent scans are preferably acquired atsubstantially the same point in the cardiac cycle (i.e., gated in thesame manner) as the background scan. However, if the T₁ of the ¹²⁹Xe inthe blood and vessels of interest is not sufficiently long to permitwaiting almost an entire cardiac cycle after ventricular systole toacquire a spectrum, it is also possible to take the background spectrumat a different point in the cardiac cycle from the subsequent spectra.Alternatively, both the background spectrum and subsequent spectra canbe acquired immediately after ventricular systole, triggered by the “T”wave of the electrocardiogram (see bottom graph of FIG. 4). Although thevolume of the ventricle is increasing during this period as shown in themiddle graph of FIG. 4, the aortic blood flow is low because the aorticvalve has closed. However, scanning immediately after systole may beless preferable in obtaining a good background scan.

As known to those of skill in the art, pulsing to produce a large flipangle (between about 45-90 degrees, and typically about 90 degrees)excitation pulse results in a better SNR (signal to noise ratio) butdestroys more polarization over small flip angle pulses. Therefore, atleast the initial spectra (i.e. before the background spectrum) shouldbe pulsed such that small flip angles (approximately 30° or less) areproduced. If a better SNR is desired for later spectra, larger flipangles can be induced in subsequent spectra once a predeterminedthreshold level of hyperpolarized Xe has been attained. Small flipangles are particularly advantageous when examining the heart becausethe heart at any time contains the blood supply soon to be distributedthroughout the body. Therefore, obviously a large flip angle willdeleteriously affect the ¹²⁹Xe polarization in the heart and therebymake it difficult to obtain spectra immediately thereafter. However, ifa single spectrum (or alternatively spectra obtained in substantiallylarge time intervals such that the hyperpolarized ¹²⁹Xe supply isreplenished) is desired, it may be advantageous to use large flip anglesto obtain the benefits of a better SNR. Co-pending U.S. patentapplication Ser. No. 09/271,476 describes the use of large flip anglepulses and pulse sequences using hyperpolarized noble gas, the contentsof which are hereby incorporated by reference as if recited in fullherein.

The block diagram of FIG. 5 outlines one method of compensating for thehyperpolarized ¹²⁹Xe signal in the heart to enhance the hyperpolarizedgas signal in the region of interest. First, a bolus of hyperpolarized¹²⁹Xe is introduced into a patient via inhalation or injection (Block500). Subsequent thereto (and preferably immediately afterwards), a MRspectroscopy signal or spectrum of the heart is obtained, the excitationsignal preferably being triggered and transmitted responsive to apredetermined cardiac event (Block 510). If a predetermined thresholdamount of ¹²⁹Xe is not present, as determined by observing the size ofthe peak representing plasma-dissolved phase ¹²⁹Xe, for example (i.e., aspectral peak with a chemical shift (compared to gas phase ¹²⁹Xe) ataround 195.6 ppm), additional spectra of the heart are acquired untilthe threshold has been reached. If the threshold of ¹²⁹Xe has beenachieved (indicating that hyperpolarized ¹²⁹Xe has reached the heart insufficient quantities), the spectrum data is saved as the “backgroundspectrum” for future spectra (Block 530). Once the threshold has beenreached and a suitable background spectrum has been obtained (Block530), the heart can be intermittently interrogated (Blocks 540 and 550),preferably initiated by the same cardiac event which was used to gatethe background scan. In certain embodiments, once all desired spectrahave been acquired, the background spectrum can be subtracted from allsubsequent spectra (Block 560) to provide corrected data.

A simulated sample background spectrum is illustrated in FIG. 11A. Asshown in FIG. 11A, the chemical shifts representing ¹²⁹Xe in the bloodand plasma are prominent. A simulated spectrum illustrating apost-background spectrum is shown in FIG. 11B. As shown, very little (ifany) difference between the two spectra (i.e. the spectra of FIG. 11Aand FIG. 11B) can be observed because of the large spectral peaksassociated with ¹²⁹Xe in the blood in FIG. 11A.

As shown in the simulated image in FIG. 11C, the subtracted correctedspectrum has a substantially smaller signal associated with the free gasphase (at 0 ppm), the plasma-dissolved phase ¹²⁹Xe (at about 195.6 ppm)and the dissolved phase ¹²⁹Xe in the red blood cells (at approximately213 ppm). Advantageously, a spectral peak at about 192 ppm representing¹²⁹Xe dissolved in atherosclerotic plaques is substantially moreprominent after background subtraction.

After background subtraction, the corrected data is processed,including, but not limited to, identifying and measuring spectral peaksof interest. Finally, the corrected spectra can be displayed (Block 580,FIG. 5) and further analyzed for the detection of pathology in vivo(Block 590, FIG. 5) such as, but not limited to, area calculation and/orratio calculation of the associated peak(s) as described herein.

Additional information can also be gathered from hyperpolarized ¹²⁹Xespectra using alternative data (besides chemical shift) and pulsesequences. For example, diseased tissues may have different diffusioncoefficients, or display unique contrast parameters such as T₂*, T₂, T₁,and/or Tip. Spectroscopy methods to observe changes in these parametersare well known to those of skill in the art. Spectroscopy skillfullyutilizing these parameters (e.g., by adeptly designing pulse sequences)and others (such as flow techniques) may facilitate targeting biologicalphenomenon of interest and/or assist background filtering.

For example, as estimated by Wolber et al, ¹²⁹Xe exchanges rapidlybetween red blood cells and plasma (on the order of about 2 to 3 ms).However, when ¹²⁹Xe is in an environment where it does not exchangerapidly, T₂ was estimated by the same group to be approximately 320 ms.Because of its lipophilic, nature, hyperpolarized ¹²⁹Xe is unlikely toleave arterial plaques readily, causing the T₂ of ¹²⁹Xe to be relativelylong in arterial plaques. In other words, the T₂ associated with redblood cells and plasma would be short, while that associated withplaques would be relatively long. Advantageously, therefore, even if thechemical shift characteristic of atherosclerotic plaques is the same orvery similar to that of red blood cells or plasma (i.e., observed nearthe same frequency), the spectral peak of interest could beadvantageously examined by utilizing a T₂-weighted sequence.

FIG. 9A depicts a simulated spectrum, assuming operation at 1.5 T andassuming that the magnitudes of the ¹²⁹Xe in red blood cells and plasmaare ten times as large as that observed in atherosclerotic plaques(primarily due to the relatively large amount of blood in the heart).The spread in frequency observed in the Fourier Transformed spectrum ofa given peak can be calculated by

${\Delta\; v} \approx \frac{1}{2\pi\; T_{2}}$Therefore, assuming 1.5 T (therefore a ¹²⁹Xe resonance frequency ofabout 17.7 MHz):

TABLE 2 Frequency shifts in ppm for a simulated atherosclerotic plaqueand blood. Chemical Amplitude T₂ Δν Δν Component Shift (relative) (ms)(Hz) (ppm) ¹²⁹Xe - gas 0 ppm 10 10 16 0.9 ¹²⁹Xe - plasma 195.6 ppm 10 353 3 ¹²⁹Xe - RBC 213 ppm 10 3 53 3 ¹²⁹Xe - plaque 192 ppm 1 100 1.6 0.09

As FIG. 9A shows, the ¹²⁹Xe chemical shift in the plaque is typicallysubstantially obscured by the signal from the plasma and the blood.Although a small peak is typically visible, it cannot be readilydistinguished from the nearby spectral peak representing ¹²⁹Xe inplasma. If this were a real spectrum and not a simulated spectrum, theinherent noise could act to substantially obscure the small shoulderassociated with the plaque that is barely discernible in FIG. 9A.

However, as shown in FIG. 9B, with a different pulse sequence, acompletely different spectrum is observed. The simulated spectrum ofFIG. 9B assumes that a T₂-weighted pulse sequence is used with a delayof about 10 ms. With the 10 ms delay, the plasma and blood resonancesare suppressed by a factor of about 28, whereas the plaque resonance ofinterest is only suppressed by a factor of about 1.1. Therefore, bytaking advantage of inherent tissue or metabolic properties, identifyingthe ¹²⁹Xe signal associated with a phenomenon or target tissue ofinterest can become more readily discernible. Of course, the signal ofinterest need not be actually displaced at all, because a computer orprogram means can perform the necessary computations/algorithms to thenidentify the value of the targeted tissue/region/condition.

In other embodiments, the methods of the present invention can use theT₁ contrast parameter to obtain relevant data regarding the pathologicalcondition. That is, the T₁ of the polarized gas may be enhanced over theT₁ in blood as it is taken up into lipid rich or other polarization lifeextending or enhancing environments. Thus, the transmittal of theexcitation RF pulse and associated signal acquisition can be performedsuch that it is delayed and obtained a period after administration ofthe polarized gas to the subject. This delay can be timed such that itis sufficient for the hyperpolarized gas in the blood to decay to a low,negligible or dissipated level while the polarized gas taken up into thetissue or cells in the region of interest still has a sufficientlyviable life to allow the spectrum to be obtained with signal data of thehyperpolarized gas only for that in the region of interest. For example,delaying the signal acquisition to about 10 seconds to 1 minute(typically about 20 seconds to 30 seconds) after administration of thehyperpolarized gas to the subject will allow the hyperpolarized gas inthe blood to decay. In certain embodiments, the hyperpolarized gas willnaturally decay in blood in about 5-6.4 seconds, while thehyperpolarized gas taken up in certain bio-environments (such as fattytissue) may be viable or retain a sufficient polarization level to allowfor spectroscopic detection for between about 10-60 seconds longer.

In certain embodiments, the polarized gas can be delivered viabreath-hold delivery as conventionally delivered for imagingapplications. In this type of delivery, a patient inhales a quantity ofhyperpolarized gas, holds his or her breath a suitable time, and thenexhales and resumes normal breathing. To take advantage of the naturalT₁ decay in the body and the longer T₁ of the hyperpolarized gas incertain bio-environments, using the T₁ as a contrast parameter toenhance the signal over non-targeted signal hyperpolarized gas signalinformation, the signal acquisition can commence while the patientexhales or after the patient resumes normal breathing. Preferably, thesignal acquisition is carried out at about 10 seconds to about 1 minute,and preferably at about 20-30 seconds, after the patient exhales thebreath-hold lung volume.

Alternatively, as known by those of skill in the art, the status of thewalls of the carotid arteries can be monitored as an indicator of thecoronary arteries in lieu of investigating the coronary arteriesdirectly. This is due to the fact that the carotid arteries oftendevelop plaques which can indicate plaque development in the coronaryarteries. Advantageously, using the carotid arteries instead of thecoronary arteries to monitor atherosclerosis obviously substantiallyreduces the background signal problem discussed hereinabove.Furthermore, a small sensitive coil can be placed directly on the neckover one or more of the carotid arteries to better transmit and detectsignals to and from the carotid arteries and blood therein. Therefore,as is true for any part of the body which is not near the heart andwhich can be interrogated by transmitting an RF pulse through a localcoil, pulses which produce large flip angles will not deleteriouslyaffect future spectra, since the signal source (hyperpolarized ¹²⁹Xe inthe blood) is continually replenished and unaffected by the RF pulsetransmitted by the local coil. Therefore, large flip angles (more thanabout 30°) can thereby advantageously be utilized in these situations toobtain the benefits of a better SNR.

The last step of used to detect pathology generally involves analysis ofthe data. The analysis evaluates one or more selected parametersassociated with the peak spectra. This can include, but is not limitedto, the peak amplitude or magnitude of the spectra of interest, theassociated line shape or line width, the area under the curve of thespectra peak or peaks of interest, and the like. In certain embodiments,the data is corrected to account for in vivo background data (e.g., thebackground data is subtracted of filtered).

The analysis, such as for the embodiment described in FIG. 5, caninclude comparing a background-corrected spectrum (or spectral peaks ofinterest within a corrected spectrum) to a predetermined referencestandard. FIGS. 10A and 10B illustrate one way to compare two spectra toobtain an indication of the extent of a diseased state. FIG. 10A depictsa simulated spectrum of a severely atherosclerotic region where plaquedevelopment is extensive. Preferably, as mentioned hereinabove, abackground spectrum has already been subtracted if this spectrum is ofthe coronary arteries to eliminate the much larger signal from the bloodin the heart chambers. As mentioned hereinabove, often (as is done inFIG. 10A) a specific cell or tissue type can be associated with aspecific chemical shift (compared to gas phase hyperpolarized ¹²⁹Xe) inthe spectrum. For example, FIG. 10A associates the peak located at 192ppm with an atherosclerotic plaque. Therefore, comparing the peaklocated at 192 ppm in FIG. 10A with that at the same chemical shift inFIG. 10B (the latter of which is a simulated spectrum of a “healthy” orsubstantially undiseased arterial condition also after subtracting thesignal associated with the blood in the heart chambers and thereforerepresentative of a “healthy reference” spectrum), clearly indicates adifference between the two conditions. Furthermore, by normalizing thepeak (using one or more of the line shape or peak magnitude, area underthe peak, or peak width) located at 192 ppm with (for example) the peakat 0 ppm (gas phase ¹²⁹Xe), a physician, computer, or program productcan compare the observed normalized peak sizes with that of a healthystandard as discussed hereinbelow. Further the presence or absence of apeak from the standard to the response signal under analysis or thedifference in magnitude of the peak (area under the curve of the spectrapeak or associated line shape or the amplitude of the peak) or otherquantifiable parameter of the spectral peak or peaks of interest can beused to diagnose or determine the likelihood of a pathological conditionunder analysis. It is noted that the use of the peak at 192 ppm wasselected for discussion purposes because this is a location associatedwith fat or fatty tissue. In operation, other chemical shift locationsmay be identified or used as appropriate to the pathological conditionor conditions under analysis.

As mentioned hereinabove, brain spectroscopy is feasible even withinhaled ¹²⁹Xe because ¹²⁹Xe easily traverses the blood-brain barrier andblood flows to the brain in quantities sufficient for spectroscopy. FIG.2 shows spectra taken from a healthy human brain in vivo.Atherosclerosis, as discussed hereinabove, is also of interest in thebrain tissue, because it can lead to strokes. Advantageously, backgroundproblems such as those encountered in imaging coronary arteries issignificantly diminished in brain spectroscopy.

Other conditions, such as Alzheimer's disease, could also benefit fromhyperpolarized ¹²⁹Xe spectroscopy of the brain. Alzheimer's disease isone of the most common causes of dementia, or loss of mental function.The disease progresses from a loss of memory to complete incapacity toperform simple tasks and care for oneself. Many people afflicted withAlzheimer's disease live for many years and eventually die frompneumonia or other diseases. Although the average lifespan of anAlzheimer patient after diagnosis is about 4 to 8 years, some patientslive for more than 20 years with the disease. Although estimates of thenumber of people afflicted with the disease range from 25 to 50 percentof the population who is 85 or older, it is clear that the percentage ofpeople afflicted with the disease doubles with every decade of life. Asthe average life expectancy increases, therefore, degenerative diseasessuch as Alzheimer's disease will become more prevalent.

Despite large amounts of research, Alzheimer's disease is still onlypartially understood. The disease begins in the entorhinal cortex andproceeds to the hippocampus and cerebral cortex and to other regions ofthe brain. The affected regions characteristically contain degeneratedand dead neurons. A true diagnosis still can only occur in an autopsywhere the characteristic neuritic plaques outside and around theneurons, as well as neurofibrillary tangles inside the nerve cells, areobserved. To date, there is no reliable, valid, early diagnostic markerfor Alzheimer's disease. Early diagnosis of Alzheimer's disease isimportant because many other diseases, such as tumors, strokes, severedepression, and thyroid problems, whose symptoms mimic those ofAlzheimer's disease, have cures or at least effective treatments whenadministered early. Additionally, the only drug on the market to treatAlzheimer's disease is significantly more effective if administeredduring the early stages of the disease. Alzheimer's disease is thereforewell suited to be detected with ¹²⁹Xe spectroscopy because the diseaseinvolves characteristic and unique changes in the tissues. ¹²⁹Xespectroscopy may detect structural changes in tissue type and display achemical shift signature, which may advantageously enable a physician todiagnose Alzheimer's disease definitively in vivo before death, andpreferably early in the progression or near the onset of the disease.

Quantifying the spectral peaks representing chemical shifts can be ofcritical importance in determining the extent of the disease forprogressive diseases such as atherosclerosis and neurological diseases(such as Parkinson's disease, multiple sclerosis, and Alzheimer'sdisease), because the stage of the disease may determine the desiredcourse and type of treatment. Therefore, for accurate quantification ofhyperpolarized ¹²⁹Xe as an indicator of disease, secondary conditionssuch as the polarization of the administered gas and the volume ofhyperpolarized gas administered to the subject are preferably accountedfor in any ¹²⁹Xe spectroscopic analysis. One quantification methodaccording to the present invention is to take the ratio of two observedspectral peaks of interest such as described hereinabove. For example,in atherosclerosis, the intimal layer decreases while the amount oflipid contained in the vessel walls increases. Therefore, a ratio of thepeaks representing the destruction of the intimal layer and theaccumulation of lipids in the media may be of clinical importance.Advantageously, this method inherently additionally factors outdifferences between people in alveolar capillary uptake of ¹²⁹Xe ifadministered by inhalation.

Alternatively, even if a single “signature” peak at a specific chemicalshift is reliably indicative of a diseased state (such as plaques in thebrain characteristic of Alzheimer's disease), a physician wouldtypically still be interested in knowing the extent of the diseasedtissue. Therefore, the present invention provides methods to quantifythe identified signature or characteristic peak(s).

In certain embodiments, to account for secondary factors, the presentinvention normalizes the peak of interest by taking the ratio of thesize of the observed peak to a second peak which is always present butnot necessarily of any interest (such as the dissolved phase ¹²⁹Xe peak(such as that representing ¹²⁹Xe in the plasma) or even the gas phase¹²⁹Xe). Therefore, a normalized value for a single spectral peak ofinterest can be obtained. Normalizing a spectral peak representing achemical shift of interest by analyzing/comparing the dissolved phase¹²⁹Xe (in the plasma or RBCs) advantageously also inhibits other factors(such as other transport deficits or metabolic disorders) fromundesirably influencing or affecting the results. Since the dissolvedphase signal in the blood (e.g. plasma) is effectively the “supply” ofhyperpolarized ¹²⁹Xe that the tissue sees, it may be appropriate todetermine the volume of hyperpolarized ¹²⁹Xe in a tissue with respect toits supply as described hereinabove.

Turning to FIG. 8, a normalization procedure according to the presentinvention can be performed after any number of spectra have beenacquired, although preferably background subtraction (if desired) isperformed prior to normalization. Data is acquired (preferably throughNMR spectroscopy as described hereinabove) of a patient's region ofinterest (Block 800). The Fourier transform of the resulting data isdetermined (Block 810), and spectral peaks at frequencies of interestare identified (Block 820). The size of a first and a second spectralpeak of interest is calculated either by magnitude or area as describedhereinabove (Block 830), and the ratio of the magnitudes and/or areas oftwo peaks is determined (Block 840). If there is only one peak ofinterest, its size can be normalized by an internal standard, such asthe size of the plasma-dissolved phase ¹²⁹Xe spectral peak, as describedhereinabove. The calculated ratio is then compared to that obtained in ahealthy person, across a population segment, or to an earlier data setfrom the same patient (Block 850). If the two ratios are notsubstantially the same, a pathological disorder can be diagnosed,identified as being likely, or deemed to have progressed or regressed(Block 860). However, if the two ratios are substantially the same, thesuspected problem is not confirmed or a change in disease status has notoccurred (Block 870) and either further evaluation for other disorders(by looking at different spectral peaks or interrogating another regionof interest) can be accomplished or the patient can be declared healthyor stable (Block 890) and/or a “baseline” can be stored for futurecomparison.

Additionally, or alternatively, another methods to normalize the data toprovide reliable quantitative signal information across subjects is tofactor out secondary conditions directly. For example, beforeadministering the hyperpolarized ¹²⁹Xe to a patient, the polarization(and preferably also volume) can be determined. In this scenario,“standards” for healthy populations or individuals could be established,for example, in units of signal per percent polarization per mL ¹²⁹Xeadministered. FIG. 7 shows one embodiment of how this technique can beused in practice. First, a polarization measurement for a known quantityof hyperpolarized gas is obtained (Block 700). Then, part or all of themeasured hyperpolarized gas is administered to a patient (Block 705).The volume of gas administered to the patient is carefully measured,either automatically (as with a ventilator) or manually (such asmeasuring the amount remaining in the container) (Block 710). At leastone data set relating to the region of interest is acquired (Block 720),and its Fourier transform calculated (Block 725). Based on the measuredpolarization and the volume of gas, the acquired spectra are normalizedto an external standard (Block 730). The spectral peaks representingchemical shifts of interest in the resulting spectra are located (Block735), and their size (magnitude and/or area) is determined (Block 740).A “healthy”/substantially non-diseased standard is then compared to theobserved results, and if the sizes are substantially the same, thesuspected ailment is not confirmed (Block 755). However, if the spectralpeaks substantially differ in size, a physician or program means canquantify the extent of the disease by observing how different theobserved spectral peaks are from the healthy/substantially non-diseasedstandard (Block 760). Preferably, if a pathological condition issuspected or confirmed, a physician will monitor the progression of thedisease over time (Block 765). As mentioned hereinabove, if backgroundsubtraction is desired, preferably it is done before locating and sizingthe peaks of interest (Blocks 735 and 740).

A polarization measurement (Block 700, FIG. 7) can be obtained in aninstrument such as a calibration station described in co-pending andco-assigned patent application identified by U.S. application Ser. No.09/163,721 to Zollinger et al., entitled “Hyperpolarized Noble GasExtraction Methods, Masking Methods, and Associated TransportContainers,” the contents of which are hereby incorporated by referenceas if recited in full herein. In this application, Zollinger et al.describes an apparatus for determining the extent of polarization for asample of hyperpolarized gas (typically proximate in time but prior toadministration). An apparatus 600 for evaluating the polarization asdescribed by Zollinger et aL is shown in FIG. 6. As shown, a quantity ofhyperpolarized gas 620 is placed in a magnetic field, preferablyprovided by a pair of Helmholtz coils 610. An NMR coil (not shown) iscontrolled by a circuit 630 and used to transmit an RF signal to thequantity of gas 620. The gas 620 responds to the RF signal and the NMRcoil detects the response and transmits it back to the circuit 630. Theresponse signal from the hyperpolarized gas is then processed by thecircuit 630 and preferably a computer 650 and display 640. The display640 can reflect the response of the hyperpolarized gas 620 to the RFpulse as shown in FIG. 6, or modified to display a calculated parametersuch as percent polarization or the Fourier transform of the gasresponse (not shown). Therefore as described in Zollinger et al, thepolarization extent can be accurately determined. Other methods ofaccurately assessing polarization are known to those of skill in theart. Preferably, a computer or program means can therefore additionallyaccount for the initial polarization level and/or the precise volume ofhyperpolarized gas administered as described hereinabove.

With the present invention as described herein, hyperpolarized ¹²⁹Xe canbe used as an extremely sensitive diagnostic probe for pathologicalconditions. More advantageously, depending on how the 129Xe isadministered, the technique is either non-invasive or minimallyinvasive. By additional manipulation (such as subtracting a backgroundspectrum) or by utilizing inherent properties of ¹²⁹Xe in variousenvironments, hyperpolarized ¹²⁹Xe may be able to additionally allow aphysician (or a computer/program means) to not only observe a biologicalactivity of interest, but be able to clearly discern it from otheractivities which are not of interest. Additionally, the presentinvention may advantageously allow a patient to be scanned for amultitude of pathologies, possibly using a whole body coil. If a wholebody coil is used, methods similar to slice-selection as known to thoseof skill in the art may be utilized to scan sections of a patientseparately and thereby obtain data from a variety of body parts withlittle inconvenience to the patient.

In progressive diseases, the present invention further advantageouslyallows a physician to quantify the extent of a pathological condition,thereby promoting appropriate and more effective treatment. In diseasessuch as cancer in which metastasis occurs though the lymph system, aphysician can furthermore-investigate a plurality of lymph nodes toassess the extent of the disease. Currently, multiple biopsies arecarried out to investigate the status of the lymph nodes, from which arecommendation of a preferred treatment regimen (e.g. chemotherapy) ismade. Utilizing hyperpolarized ¹²⁹Xe to perform this interrogation couldbe substantially more convenient and less painful for the patient.Further advantageously, the methods of the present invention can be usedto quantitatively assess the effectiveness of a treatment regimen.

Obviously, although specific pathological conditions such as Alzheimer'sdisease and atherosclerosis were discussed, the present invention can beused for any disease which is typically marked by a change which can bedetected in vivo with hyperpolarized ¹²⁹Xe NMR spectroscopic analysis,including, but not limited to, changes in cell and/or tissue compositionthat can be detected directly by observing chemical shifts (compared togas phase ¹²⁹Xe). In so doing, the spectroscopic analysis can be carriedout based on one or more physical contrast parameters of the gas such asD, T₁, T₂, T₂*, and Tip.

The following non-limiting example is provided in further description ofthe present invention.

EXAMPLE

Xenon-129 NMR Spectroscopy in Human Aortic Tissue

Background:

The well-established property of the ¹²⁹Xe nucleus to respond to changesin the environment with a change in NMR-resonance frequency led to aproposal that the lipid-rich atherosclerotic plaques in humans could bedetected by measuring the chemical shift of the xenon signal. ¹²⁹Xespectra have been recorded in human tissue at 7 T and 25 bars ofpressure. The ¹²⁹Xe was thermally polarized for this experiment.

Method:

Samples of aortic wall were taken from recently deceased humans,unfrozen but kept on ice. The innermost layer of the aortic wall, theintima, was dissected by lifting by tweezers and gently cutting it freewith a sharp scalpel. The tissue was sliced into pieces of a few mm.Then, for each patient, a control part of “healthy” vessel wall wastaken and compared to plaques cut out the same way. The plaques werescored on a graded scale ranging from 1-5 with grade 1 representingtotally soft, fatty deposits, grade 3 moderate calcification, and grade5 very hard completely mineralized deposits.

The samples were packed loosely in a heavy walled sample tube of 10 mmouter diameter and 4 mm inner diameter fitted with steel tubing and avalve. A piece of glass wool was used as a spacer to center the samplein the detection coil. The samples were pressurized with 25 bar of 80%thermally polarized ¹²⁹Xe (remainder other Xe isotopes) withoutfreezing. The sample tube was then transferred to an NMR spectrometerand ¹²⁹Xe spectra were recorded at 7 T at a resonance frequency of 82.98MHz and at a temperature of 37° C. The chemical shifts are given in ppmwith the ¹²⁹Xe gas peak set to 0 ppm.

Results:

The significant results are summarized in Table 3 below. The conclusionis that there is a difference in chemical shift of slightly less than 2ppm (1.5-1.9), which can be deemed significant in a spectrometer of thequality used in this study.

TABLE 3 Cause of Healthy Plaque Difference Sample no. Age Sex deathPlaque, grade (ppm) (ppm) (ppm) 1 70 F Suffocation 2-3 187.4 188.9 1.5 255 M Cardiac 4-5 189.2 187.29 1.9 infarctIt is anticipated that for in vivo applications, the signal can beobtained, enhanced or optimized by certain of the techniques describedabove to help amplify the chemical shift associated with thepathological condition of interest. That is, the signal strength can beinfluenced by the degree of polarization of the ¹²⁹Xe at administration,the time it takes the polarized gas product to reach the region ofinterest for the pathology under evaluation, and how much of thepolarization/gas is taken up by the pathology under evaluation, and theselected pulse sequence and timing thereof (relative to administrationof the polarized gas). For example, the pulse sequence can be adjustedto suppress or enhance the desired spectra in the response signal suchas by selecting a weighted T₂ or delayed T₁ contrast parameter. For aparticular, condition, a line shape, peak amplitude, area under the peakor peaks of interest, peak-to-peak ratio (or ratio of the area underthose peaks), or other desired value at one or more chemical shiftregions may be representative of the disease. For example, foratherosclerotic plaque, if the plaque is at a stage where it is lipidrich with (fatty unstable) lesions, the signal may be increased (andcentered over a different chemical shift location) over late stagehardened plaque.

In the results presented above, the chemical shift is up (from 187.4 to188.9) for the female sample and down (from 189.2 to 187.29) for themale sample. This shift variation may be attributed to the plaque grade,or may be associated with gender and/or age. It is also possible thatthe “healthy tissue” was at an earlier stage of the disease, but perhapsnot without some plaque. Another variable to consider in the dataobtained in this study is the possibility in the variation of pressurein the volume of the vessel/tissue on the signal for the samples. Inaddition, when comparing to the discussion for FIG. 10, the location ofthe signal spectra location in this study should be adjusted up by about11 ppm to account for the increase in pressure. That is, high pressuresshift the gas resonance. Thus, for sample no. 1, the plaque ppm at 188.9generally corresponds to a 199.9 ppm.

FIGS. 12A and 12B illustrate the spectrum for the male samples discussedabove. The chemical peak for the tissue of interest illustrates that notonly is there a detectable shift, the line shape and/or peak(s) may alsobe different (one peak present in either the “normal” or “healthy”tissue may be reduced, shifted, or absent in the other tissue). Thesedifferences can be analyzed by statistical curve fitting algorithmsknown to those of skill in the art. FIGS. 13A and 13B illustrate asimilar alteration in line shape, shift, and peak magnitude, for thefemale samples.

The foregoing is illustrative of the present invention and is not to beconstrued as limiting thereof. Although a few exemplary embodiments ofthis invention have been described, those skilled in the art willreadily appreciate that many modifications are possible to the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe claims. In the claims, means-plus-function clause are intended tocover the structures described herein as performing the recited functionand not only structural equivalents but also equivalent structures.Therefore, it is to be understood that the foregoing is illustrative ofthe present invention and is not to be construed as limited to thespecific embodiments disclosed, and that modifications to the disclosedembodiments, as well as other embodiments, are intended to be includedwithin the scope of the appended claims. The invention is defined by thefollowing claims, with equivalents of the claims to be included therein.

1. A method for detecting atherosclerosis suitable for in vivoevaluations comprising: administering a first bolus of hyperpolarized¹²⁹Xe gas in vivo to a patient so that said hyperpolarized ¹²⁹Xe travelsto a region of interest; applying a first at least one excitation pulseto said first administered hyperpolarized ¹²⁹Xe in the region ofinterest; acquiring at least one first response signal spectrumpresenting a response of said hyperpolarized ¹²⁹Xe to said at least onepulse via nuclear magnetic resonance spectroscopy; identifying at leastone spectral region of interest in said first response signal spectrum;analyzing said at least one spectral region of interest in said firstresponse signal spectrum; determining the presence of atheroscleroticplaques on the basis of the presence or absence of at least one spectralpeak and/or at lest one selected feature of at least one spectral peakin the response signal spectrum based on said analyzing and identifyingsteps; providing a therapeutic agent to the patient to treat theatherosclerotic condition; then administering a second bolus ofhyperpolarized ¹²⁹Xe gas in vivo to a patient so that saidhyperpolarized ¹²⁹Xe travels to a region of interest; applying a secondat least one excitation pulse to said second administered hyperpolarized¹²⁹Xe in the region of interest; acquiring at least one second responsesignal spectrum representing the response of said hyperpolarized ¹²⁹Xeto said at least one pulse sequence via nuclear magnetic resonancespectroscopy; identifying at least one spectral region of interest insaid second response signal spectrum; analyzing said at least onespectral region of interest in said second response signal; andevaluating the treatment efficacy of the therapeutic agent based on thefirst and second analyzing steps.
 2. A method according to claim 1,wherein said evaluating step comprises comparing selected spectralcharacteristics in the first and second response signal spectrums.
 3. Amethod according to claim 1, wherein said second analyzing step furthercomprises identifying whether the atherosclerotic condition hasdeteriorated or progressed from the time of the first analyzing step. 4.A method according to claim 1, further comprising evaluating at leastone of the acquired first and second response signal spectrums toidentify whether the atherosclerosis is likely to correspond to an earlystage with soft, fatty deposits, an intermediate stage with moderatecalcification, or a late stage with hard mineralized or calcifieddeposits and/or lesions.
 5. A method for detecting atherosclerosisaccording to claim 1, wherein the region of interest comprises at leastone of the carotid arteries.
 6. A method for detecting atherosclerosisaccording to claim 5, wherein said at least one excitation pulse for thefirst and second applying steps comprises a resonant RF pulse sequenceselected to produce large flip angles in said hyperpolarized ¹²⁹Xe.
 7. Amethod according to claim 1, wherein the first applying, acquiring andanalyzing steps are carried out to define a baseline physiologicalprofile of the patient.
 8. A method according to claim 1, wherein atleast said first analyzing step comprises assessing whether the width ofa curve associated with a peak in the response spectrum is narrower thana corresponding peak in a reference spectrum corresponding to anepidemiological study and/or an a priori baseline spectrum to determinewhether the patient has atherosclerosis.